I couldn't see any reason why it needs such a containment, can't you just say "a large Euclid containment unit" and be done with it?

As of Addendum XXXX-EU, Visual recordings of SCP-XXXX being played are to be destroyed, and the personnel responsible are to undergo pyschological evaluation.

Pretty sure you don't need "visual" to be capitalized after the first comma. "the personnel responsible are to undergo pyschological evaluation." Responsible for what? Having visual records, making visual records, or just playing it?
"pyschological"
*psychological

The object appears to have no anomalous appearance, with the exception of two candlesticks, which are fixed to the object's mantlepiece.

The candlesticks don't to have much to do with anything about the anomaly.

Personnel are permitted to enter SCP-XXXX's containment chamber once a week for a performance, and are allowed to make recordings of the music, as there has been no known adverse affects towards doing so.

Just because it's cross-out doesn't give it an excuse to be incorrectly formatted, this should be in the containment procedures

In visual recordings, subjects report seeing "apparitions" in a state of extreme trauma.

Is there a reason "apparitions" is in quotes?

The addendum feels pointless, I mean it's kind of creepy but I don't care enough to really trigger an emotion. It does the plot-line of "object does this thing and just this thing. PLOTWIST! It's dangerous!" I'm not saying you can't do this plot-line, it's just that with this attempt that's all it is. No additional emotions, an SCP needs to be super interesting or trigger some significant emotion that makes us think and feel, the best scips do both of these.

E: added a collapsible, the post turned out to be bigger than I thought it would be.

When a performance on SCP-XXXX is recorded, the effect is polarized into a sound described as "The Dissonance of [REDACTED]"

This sentence is a bit weird. Something that is polarized is divided into two sharply contrasting effects. Since there's only one sound here, it can't actually be polarized. In addition, are all subjects actually stating that the sound is described as 'the dissonance of [REDACTED]'?

In visual recordings, subjects report seeing apparitions in a state of extreme trauma.

Clarify the sentence as 'video recordings of performances'. Also, how do the subjects report seeing the apparition if the performance is being recorded? Are they watching their recorded performances played back? You should also clarify that the apparitions are of human beings.

Audio recordings result in the subject immediately shattering the eardrum, as well as attempting to [REDACTED]

Do you mean the subject shatters their own eardrum? And is it actually the subject popping their own ears or the recording shattering the subject's eardrums?

Within 1-5 minutes, an affected individual will attempt to tear itself apart, upon doing so, the remains will slowly liquify into saline solution

'Themselves' not 'itself'. The second comma between 'apart' and 'upon' should be replaced by a semicolon.
Also, it's pretty difficult for a person to tear themselves apart. Maybe you could specifically describe how they do so to add some spicy body horror.

The process is currently unknown, as it has only occurred outside of monitoring.

Then how was the process described in the document?

The death of an affected subject will correspond to the addition of a new string, as well as the replacement of the candles with new ones.

If a new string is added to the piano, where does it go? Does the piano expand to make room or something?

THOUGHTS:
The object overall is fairly boring: a pretty piano that makes people kill themselves and melt. Plus, you have contradictory descriptions, missing information, and the plot thread with the ghosts goes literally nowhere. The core issue right now is that your idea is not interesting: it is an object that, put simply, makes someone die. That's it.

There's some interesting places you could take this, though. What is the purpose of the apparitions? Why do they exist? Are they people who were killed by the piano and are trying to warn the next player? When writing an SCP, I advise trying to come up with a story and then figuring out how to write an SCP around that story.